X-ray powder diffraction characterization of the elusive ...

X-ray powder diffraction characterization of the elusive tetraphosphine
Si„CH 2 PPh 2 … 4 silane
Norberto Masciocchi and Simona Galli
Dipartimento di Scienze Chimiche e Ambientali, Università dell’Insubria, via Valleggio 11,
22100 Como, Italy
Mona Bogza and Janet Blümel
Department of Organic Chemistry, University of Heidelberg, Im Neuenheimer Feld 270,
D-69120 Heidelberg, Germany
Received 12 September 2006; accepted 1 December 2006
X-ray powder diffraction data for the tetraphosphinic SiCH 2 PPh 2 4 silane are reported. Its crystal
and molecular structures were determined by simulated annealing and full-profile Rietveld
refinement methods. SiCH 2 PPh 2 4 was found to have tetragonal symmetry with P-42 1 c space
group. The lattice parameters were determined to be a=17.2112 Å, c=7.5531 Å, V
=2237.55 Å 3 . The crystal structure was found to contain isolated SiCH 2 PPh 2 4 molecules. In
each SiCH 2 PPh 2 4 molecule, the central Si atom was fixed at the −4 symmetric position bearing
four CH 2 PPh 2 branches. This environment was confirmed by 31 P CP/MAS NMR measurements.
Thermo-diffractometric measurements in the 20–120 °C range were also used to estimate the linear
and volumetric thermal expansion coefficients ln V/T=1.810 −4 K −1 , typical for very “soft”
materials. © 2007 International Centre for Diffraction Data. DOI: 10.1154/1.2424476
Key words: powder diffraction, silane, phosphine, Rietveld refinement
I. INTRODUCTION
Chelating phosphines incorporating ethoxysilane groups
are used successfully as linkers for immobilizing catalysts on
oxide supports in order to improve their recyclability and
lifetime. During the preparation of several phosphines of the
type EtO x SiCH 2 PPh 2 4−x x=1–3, Ph=C 6 H 5 Bogza
et al., 2005, we also synthesized the tetraphosphine
SiCH 2 PPh 2 4 in high yields from cheap starting materials.
SiCH 2 PPh 2 4 has the potential to act as a multidentate
bridging ligand for different metals, allowing the preparation
of organometallic polymers or dendrimers incorporating
metal complexes, as shown for the analogue
SiCH 2 CH 2 PPh 2 4 Hendriksen et al., 1989; Ropartz et al.,
2002. While the ethoxy derivatives could eventually be prepared
as single crystals and were characterized by conventional
X-ray diffraction structure methods Bogza et al.,
2005, SiCH 2 PPh 2 4 was only obtained as a colorless polycrystalline
material. In this paper, SiCH 2 PPh 2 4 was characterized
by ab initio X-ray powder diffraction XRPD methods
Masciocchi et al., 2005 and multinuclear NMR
spectroscopy in solution 1 H, 13 C, 29 Si, and 31 P, 31 P CP/
MAS NMR in the solid state, and high-resolution FAB
mass spectrometry.
II. EXPERIMENTAL
Ph 2 PCH 2 Li·TMEDA 1.76 g, 5.44 mmol was dissolved
in a mixture of pentane 10 ml and THF 5 ml, and
then treated with SiCl 4 0.23 g, 1.36 mmol. Stirring the reaction
mixture for 2 h at RT caused the precipitation of a
colorless material, which was filtered, washed with pentane,
and dried in vacuo. Yield: 0.74 g, 0.89 mmol, 64.5%. Relevant
spectral data can be found in Bogza et al. 2005.
Diffraction data were recorded on a Bruker AXS D8
Advance diffractometer operating in the : mode, equipped
with a secondary beam graphite monochromator, a NaTlI
scintillation counter, and pulse-height amplifier discrimination.
CuK radiation =1.5418 Å was used. The X-ray
generator and diffractometer settings were 40 kV, 40 mA,
DS 0.5°, AS 0.5°, and RS 0.1 mm. Carefully ground powders
were deposited on a quartz zero-background plate. Experimental
conditions were step scan mode, with 52105°,
2=0.02°, and t=30 s step −1 . Silicon NBS 640b was used
as an external standard. The recorded pattern shown in Figure
1 could not be matched with any of the phases reported
in the Powder Diffraction File ICDD, 2005.
Peak search methods and indexing of the first 16 lines
227° by TOPAS-R Bruker AXS, 2005 allowed the
determination of a primitive tetragonal cell with approximate
lattice parameters a=17.20, c=7.54 Å, V=2231 Å 3 , and
GoF=53.1. A survey of the 2006 version of the Cambridge
Structural Database showed no match of any compound with
these lattice metrics or one derived therefrom by standard
cell-reduction programs. We therefore decided to solve the
structure by ab initio XRPD methods using the experimental
data shown above and the assigned hkl indices are reported
in Table I.
Systematic absences, density, and geometrical considerations
indicated Z=2 and among others space group
P-42 1 c, later confirmed by successful structure solution and
refinement. The structural model employed in the final
whole-pattern Rietveld refinement was determined ab initio
by the simulated annealing technique implemented in
TOPAS-R; the silicon atom was fixed at the cell origin and
partially flexible rigid bodies for the phosphine branches
were used, helping convergence and stability, with three refinable
conformational parameters: the C-P-C-Si and two
C-C-P-C torsional angles. The background contribution was
55 Powder Diffraction 22 1, March 2007 0885-7156/2007/221/55/4/$23.00 © 2007 JCPDS-ICDD 55

Figure 1. Raw diffraction data for SiCH 2 PPh 2 4 in the 5260° range.
modelled by a polynomial function, no preferred orientation
correction was found to be necessary, and a single isotropic
B M value was refined. Peak shapes were described by the
fundamental parameters approach with anisotropic peak
broadening. The final Rietveld refinement plot and a sketch
of the crystal and molecular structures are shown in Figures
2 and 3, respectively. Table II contains the relevant crystal
data and data analysis parameters, and Table III contains the
final fractional atomic coordinates.
TABLE I. X-ray powder diffraction data for SiCH 2 PPh 2 4 .
2 obs deg I/Io h k l d obs Å d cal Å
7.263 100.0 1 1 0 12.1615 12.1544
10.286 1.3 0 2 0 8.5930 8.5945
12.827 1.1 0 1 1 6.8959 6.9093
14.559 5.8 2 2 0 6.0793 6.0772
16.452 62.0 2 1 1 5.3837 5.3850
18.600 1.0 3 2 0 4.7666 4.7674
20.126 82.0 3 1 1 4.4086 4.4105
20.660 4.1 0 4 0 4.2957 4.2972
21.316 1.8 4 1 0 4.1650 4.1689
22.037 28.0 3 2 1 4.0304 4.0304
23.138 3.2 4 2 0 3.8410 3.8436
23.809 4.5 0 4 1 3.7342 3.7342
24.379 18.0 4 1 1 3.6482 3.6491
24.699 5.4 1 1 2 3.6017 3.6033
25.862 5.3 0 2 2 3.4423 3.4547
26.042 4.1 4 2 1 3.4189 3.4249
26.388 3.4 5 1 0 3.3749 3.3710
27.837 4.2 2 2 2 3.2024 3.2054
28.803 3.5 3 1 2 3.0971 3.0994
30.204 6.0 3 2 2 2.9566 2.9585
31.995 4.7 4 1 2 2.7951 2.7974
32.511 3.7 5 3 1 2.7518 2.7458
6 2 0
33.303 4.7 4 2 2 2.6882 2.6925
33.880 3.6 6 1 1 2.6438 2.6464
35.455 3.5 5 4 1 2.5298 2.5292
35.681 4.6 5 1 2 2.5143 2.5138
36.089 2.1 0 1 3 2.4868 2.4888
37.008 2.2 6 3 1 2.4271 2.4263
37.711 1.2 6 4 0 2.3835 2.3837
38.580 1.5 0 7 1 2.3318 2.3350
Thermodiffractometric experiments were performed in
air in the 10–120 °C range using a custom-made sample
heater with nominal ±0.1 °C stability, assembled by Officina
Elettrotecnica di Tenno, Ponte Arche, Italy.
III. DISCUSSION
The crystals of SiCH 2 PPh 2 4 contain isolated molecules,
and each SiCH 2 PPh 2 4 molecule has one central Si
atom located at the −4 symmetric position, the origin of the
unit cell of the acentric P-42 1 c space group see Figure 3.
This feature is also shared by the SiCH 2 X 4 X=Cl Daiss et
al., 2004 and Br Ilg et al., 2006 and SiAr 4 species Ar
=Phenyl Claborn et al., 2002, Pyrrol-l-yl Frenzel et al.,
1995, 2-Furan-2-yl Neugebauer et al., 2000. At variance,
SiCH 2 OH 4 Ilg et al., 2006 and CCH 2 OH 4 Semmingsen,
1988, but not CCH 2 Br 4 , P2 1 /c Klaeboe et al., 1986,
crystallize in different tetragonal space groups, although
maintaining crystallographically imposed S 4 symmetry. Consistently,
all four CH 2 PPh 2 branches share identical environments,
so that in the 31 P CP/MAS NMR spectrum only one
sharp singlet appears at −29.6 ppm versus 85% H 3 PO 4 ,
i.e., at slightly higher field than the EtO x SiCH 2 PPh 2 4−x
x=1,2 derivatives Bogza et al., 2005. Despite the typical
low accuracy suffered by the individual geometrical parameters
bond distances and angles derived from XRPD data,
the observed coordination at the central atom is rather well
defined Si-C1 1.8483 Å, C1-Si-C1 108.02–
110.21°, matching single crystal data results of the tetrahedral
species cited above.
As for other nearly globular species, the crystal packing,
dictated by weaker, mostly H¯H intermolecular contacts,
reflects archetypic structures, which in this case, can be retraced
to the body-centered cubic one. Worthy of note, the
crystal structure of the title compound, determined by the
relative arrangement of the different molecules and by the
ordered disposition of their four branches, is acentric, but
neither polar nor chiral and reflects the S 4 symmetry of the
isolated molecule.
On measuring the powder pattern of SiCH 2 PPh 2 4 at
different temperatures 10–120 °C with 10 °C steps, we
did not observe any discontinuity in the cell volume nor in
the a and c cell parameters, which would be necessary if a
dehydration process were active; thus, there is no manifestation
of water in the lattice cavities located near 0,0, 1 4 which
56 Powder Diffr., Vol. 22, No. 1, March 2007
Masciocchi et al. 56

Figure 2. Final Rietveld refinement plot with peak markers and difference plot at the bottom.
PLATON Spek, 2005 estimated as 415 Å 3 , with shortest
interactions of 3.45 Å—H atoms excluded. A nearly ideal
linear behavior for this temperature dependence is shown in
Figure 4. From the cell parameters thus obtained, the linear
and volumetric thermal expansion coefficients could be derived
ln a/T=4.310 −5 K −1 , ln c/T=9.910 −5 K −1 ,
and ln V/T=1.810 −4 K −1 , showing values typical for
very “soft” materials. For comparison, near RT, ln V/T
=1.810 −4 K −1 for liquid mercury, 2.210 −4 K −1 for
solid sulphur, and 4.810 −4 K −1 for glycerine Perry et al.,
1998.
IV. CONCLUSIONS
In the absence of single crystals, the structure of
SiCH 2 PPh 2 4 , a recently published member of the polysubstituted
SiCH 2 X 4 class, was determined by employing
uniquely conventional laboratory X-ray powder diffraction
data and state-of-the-art ab initio techniques. The final model
was eventually refined, in P-42 1 c, by the Rietveld method,
using rigid groups with flexible conformational freedom one
C-P-C-Si and two C-C-P-C torsion angles. This work
completes the structural characterization of the
EtO x SiCH 2 PPh 2 4−x series and allows a detailed comparison
with simpler tetrasubstituted ECH 2 X 4 E=C, Si derivatives.
Moreover, with the aid of a custom made sample heater,
the temperature dependence of the lattice parameters was
also determined, which indicated a 2% increase in volume
upon heating from 10 °C up to 120 °C. This large value,
typical for nonrigid molecular solids, is likely related to a
dynamic process involving large variations of the intra- and
intermolecular contacts, eventually leading to reorientation
of the globular molecules well before melting. Work can be
anticipated in the direction of studying the thermal behavior
of SiCH 2 PPh 2 4 at higher temperatures, coupling TG and
DSC observations to HT diffraction data.
TABLE II. Crystal data and data analysis parameters for SiCH 2 PPh 2 4 .
Figure 3. Schematic drawing of SiCH 2 PPh 2 4 , viewed down 001. In each
molecule, the Si atom is the unique central atom bearing four CH 2 PPh 2
branches, and P atoms are those connected to a methylene and two phenyl
groups. Hydrogen atoms are shown as white spheres. Top: the molecular
structure left, ball and sticks; right, space filling models; bottom: the crystal
packing. Relevant geometrical parameters: Si–C1 1.8483 Å, C1-Si–
C1 108.02–110.21°, Si–C1-P–C2 179.45, Si–C1-P–C8
67.44°.
Formula
C 52 H 48 P 4 Si
fw, g mol −1 824.87
System
Tetragonal
Space group
P−42 l c
a, Å 17.2112
c, Å 7.5531
V, Å 3 2237.55
Z 2
, gcm −3 1.224
F000 868
2 range 5–105
R p , R wp 0.114, 0.146
R Bragg 0.056
GoF 3.06
57 Powder Diffr., Vol. 22, No. 1, March 2007 X-ray powder diffraction characterization of the elusive tetraphosphine ... 57

TABLE III. Fractional atomic coordinates for SiCH 2 PPh 2 4 .
Atom x/a y/b z/c
Si 0 0 0
P 0.102 06 0.093 23 0.293 64
C 1 0.019 40 0.084 71 0.143 76
C 2 0.090 51 0.188 63 0.396 33
C 3 0.130 69 0.212 74 0.544 71
C 4 0.120 75 0.287 60 0.608 92
C 5 0.069 72 0.337 36 0.528 34
C 6 0.028 08 0.313 65 0.384 78
C 7 0.038 02 0.239 19 0.317 57
C 8 0.191 94 0.100 18 0.168 23
C 9 0.197 83 0.155 09 0.037 47
C 10 0.264 77 0.161 01 −0.062 57
C 11 0.325 36 0.112 23 −0.035 75
C 12 0.320 51 0.056 98 0.092 06
C 13 0.254 44 0.051 35 0.195 79
H 1A 0.009 38 0.132 35 0.082 56
H 1B −0.025 38 0.073 59 0.211 00
H 3 0.160 83 0.180 64 0.590 29
H 4 0.147 51 0.298 29 0.724 52
H 5 0.067 02 0.387 46 0.572 07
H 6 −0.007 21 0.349 14 0.331 18
H 7 0.010 77 0.221 21 0.204 54
H 9 0.155 93 0.190 51 0.018 02
H 10 0.266 93 0.198 14 −0.135 36
H 11 0.372 70 0.116 93 −0.098 59
H 12 0.366 66 0.022 63 0.111 41
H 13 0.245 90 0.009 84 0.282 23
Figure 4. Temperature dependence of the unit cell volume of SiCH 2 PPh 2 4 ,
in the 10–120 °C range.
ACKNOWLEDGMENTS
The authors are indebted to the Fondazione Provinciale
Comasca and the Deutsche Forschungsgemeinschaft SFB
623 for financial support. We also thank Professor Angelo
Sironi University of Milan for helpful discussions.
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